The Mega Atlas contains towns that are not in the Kepler atlas. The Mega Atlas does NOT replace the Kepler or Sirius Atlas because it contains only towns that are not in the Atlas.
The Mega Atlas provides over 1½ million additional places. Many of these places are very small towns and villages, and some of them are alternative names of towns, such as when a town has a change of name. Birth places of historical figures are sometimes given as towns which many no longer exist or have had a change of name and in some cases this previously existing town or town with a different name will appear in the Mega Atlas.
If you enter the birth place into Kepler or Sirius and the town is not found in Kepler's or Sirius's Atlas, you can click on the Mega Atlas button to see if it is in the Mega Atlas. The Sirius and Kepler Atlas contains over 180,000 towns and nearly every town that a person is born in is already in it but for those unusual cases when the town is not contained in Kepler or Sirius, the Mega Atlas is very helpful.